This disclosure is directed to an apparatus and procedure for treatment of film emerging from a film forming device. The film is typically formed continuously and has a specified width. It is not uncommon to form film in the range of about six mils thick or less, typically from polymeric materials. A typical material is polyethylene which is formed into the film material. Obviously, alternate polymers or copolymer systems can be used to manufacture continuous output film supplied in specified width and thickness and which typically emerges from the machine at a specified rate or velocity.
U.S. Pat. No. 3,065,098 sets forth multiple rollers having perforations for undulating a web of material for drying. An alternate structure is a sieve drum set forth in U.S. Pat. No. 3,585,729. A suction draft accompanies the sieve drum. Moreover, U.S. Pat. No. 4,364,185 shows a rotating cylinder 12. Hot gas flowing at an exemplary rate of 5,000 cfm is described in conjunction with the rotating cylinder. It is submitted that the structures described above, representative of the prior art, do not set forth a method for achieving surface fluoridation of polymer film. Even more remote are the procedures for fluorinating blow molding polymers as exemplified in U.S. Pat. No. 4,396,567; this is typically applying gas within a fabricated bottle, jug, etc.
This apparatus and procedure accomplishes a surface fluoridation of a polymeric film. As will be understood, the extrusion fabrication procedure orients long polymer molecules lengthwise (with the extrusion direction) to thereby form the film. The physical or mechanical characteristics of the film are in large part determined by the particular polymer system utilized in fabrication of the film; this method and apparatus enables the film to be modified by application of fluorine to the surface, thereby achieving change in surface of the polymeric film. As will be understood, in a typical thin film (up to about six mils thickness), it is not necessary or desirable to accomplish fluoridation fully through the extruded plastic mass. (Rather, fluoridation of the polymer surface (or cross-linking) can be implemented at surface molecules.) Thus, a change in the skin properties and characteristics typically achieves significant physical property changes and thereby enables the film to obtain significant gains in mechanical characteristics for a relatively small cost increment. As an example, one should consider the relative cost per unit weight of low density polyethylene film in contrast with the cost of more sophisticated polymer or copolymer systems (e.g., fluorinated ethylene--propylene). A modification thus accomplishes substantial change in mechanical characteristics without the excessive cost that would be associated with more expensive fully fluorinated polymer or copolymer systems. Moreover, the method of manufacture of low density polyethylene entails a less costly and less difficult manufacturing process.
Many products operate quite satisfactorily with only a surface treatment. For instance, surface treatment by the procedure described hereinbelow changes the wettability of polyethylene film. There is no need to extend the fluoridation treatment into the film because this is primarily a surface treatment, and many uses and applications are available for a low density polyethylene film having improved surface wettability characteristics.
With the foregoing in view, this apparatus and procedure are summaried as a method of modification of the surface of a continuously extruded film wherein the film from a film forming device is directed through a closed cabinet or housing evacuated by means of modest vacuum flow. Within the housing, there are guide rollers for directing the film around a roll. The roll is preferably formed of a sintered metal and a gas flow (a mixture of elemental fluorine and nitrogen for dilution) is directed into the roll. It flows through the roll to impinge on the surface of the film moving therepast. Moreover, the film is looped around the roll within a closed cabinet or housing which is connected with an exhaust pump to remove surplus fluorine. As desired, the equipment can be duplicated for fluorination of both sides of the extruded film. The film extends from this device to a suitable tensioning device which draws the film through the system for subsequent use or spooling.
A gas distribution system is included which mixes fluorine and nitrogen and supplies the gas through a regulator. It is delivered to the sintered roll, at a slight positive pressure. The exposure time of the film in the housing is sufficient to accomplish fluorination.